1. Field of the Invention
The present invention relates to an electronic device, especially, a ferroelectric device, a surface acoustic device, and a piezoelectric device, based on a perovskite-type oxide thin film grown epitaxially on an oxide thin layer on an inorganic amorphous layer or an organic solid layer, and an electronic apparatus comprising the electronic device.
2. Description of the Related Art
Conventionally, when a perovskite-type oxide thin film is grown epitaxially, a single crystal substrate or a crystallized buffer layer is chosen by considering lattice matching with a perovskite-type oxide to be grown epitaxially. For example, for a surface acoustic device using a perovskite-type oxide thin film, as described in Japanese Unexamined Patent Application, First Publication No. Hei 2000-278084, good properties have been obtained by growing a potassium niobate (described as “KNbO3” hereafter) thin film having a high electromechanical coupling factor (described as “k2” hereafter) on a single crystal substrate of strontium titanate (described as “SrTiO3” hereafter) with (110) orientation epitaxially. Also, as described in Japanese Unexamined Patent Application, First Publication No. Hei 9-100125, an example is presented of obtaining a KNbO3 film with (110) orientation on an amorphous substrate.
On the other hand, for a piezoelectric device such as a ferroelectric memory device or an actuator, as described in Japanese Unexamined Patent Application, First Publication No. Hei 8-330540 or Japanese Applied Physics Lett, 70, 1378-1380 (1997), attempts have been made to fabricate a device by using a thin film grown epitaxially or grown with an orientation on a crystal substrate in order to improve their properties.
Generally, a polarization direction is determined by a material, so that an epitaxial film with a controlled polarization direction is preferable in order to control device characteristics. Also, an epitaxial film is advantageous for reducing the effects of grain boundaries on the device characteristics. Furthermore, an ultra-thin film will be required for a ferroelectric memory element or a ferroelectric memory device having a ferroelectric memory cell array, which is an assembly of ferroelectric memory elements. From this viewpoint, an epitaxial film is also advantageous in order to reduce leakage current or to improve a square-loop characteristic A mere oriented film is inferior to an epitaxial film in these points. In an oriented film, packing density is tower than that in an epitaxial film, in-plane orientation is not as uniform, and crystallization is not as fully controlled. Consequently, in recent years, an epitaxial film of a perovskite-type oxide is receiving serious attention for fabricating devices.
Conventional technologies for growing perovskite-type oxide thin films epitaxially have problems described below.
First, as described above, when a perovskite-type oxide thin film is grown epitaxially, a single crystal substrate or a crystallized buffer layer having a lattice matching with a perovskite-type oxide must be used.
Even though oxide ceramics such as SrTiO3 are normally used as a single crystal substrate, the cost of oxide ceramic substrates is generally high, and fabrication of large substrates of 3 inches or more in diameter is difficult depending on the types. On the other hand, as described in Japanese Unexamined Patent Application, First Publication No, Hei 8-330540, examples are shown of epitaxially growing a perovskite-type oxide thin film via a buffer layer crystallized on a silicon substrate, which is inexpensive and suitable for large diameters. However, when a capacity increase or a miniaturization is attempted by integrating a ferroelectric memory device or surface acoustic device on a semiconductor device, usually, these must be formed on a silicon oxide amorphous interlayer insulating film of the semiconductor device; therefore, a technology to grow a perovskite-type oxide thin film epitaxially on an amorphous layer is required.
Even though a technology to grow a perovskite-type oxide thin film epitaxially on a silicon substrate is highly valuable in terms of cost and other factors, the technology alone cannot accomplish the desired integration with semiconductor devices. Also, when a piezoelectric device such as an actuator is fabricated, a perovskite-type oxide thin film must be formed on a thermally oxidized film of a silicon oxide as well, so that a technology to form a perovskite-type oxide thin film on a silicon substrate alone is not sufficient. There are examples of forming perovskite-type oxide thin films on a crystallized film of platinum or the like formed on an amorphous silicon oxide film; however, even though oriented films can be obtained in such cases, epitaxial films cannot be obtained.
As described above, an epitaxial film is superior to an oriented film for device performance. Therefore, if a perovskite-type oxide thin film could be formed on an amorphous layer, integration with a semiconductor device would become possible and not only could a high performance device be built, but a perovskite-type oxide thin film could be grown epitaxially on any substrate regardless of whether or not the substrate is crystal, thereby expanding the application range enormously. That is, any substrate could be used as long as it can withstand a temperature for forming a perovskite-type oxide thin film.
As described in Japanese Unexamined Patent Application, First Publication No. 9-100125, there is an example of forming KNbO3 on an amorphous substrate; however, what is obtained is an oriented film, and obtaining an epitaxial film is still difficult. It is difficult, furthermore, to grow a crystal in a desired orientation direction.